1. Field of the Invention
The present invention relates generally to satellite based signal antenna systems, and more particularly relates to a satellite based antenna for generating or receiving multiple transmission beams.
2. Description of the Prior Art
In satellite signal transmission systems, it is often desirable to send multiple transmission signals from a single antenna. This provides for increased signal throughput and/or increased signal coverage area. An array antenna, or an antenna using multiple antenna feed elements and a focusing device is conventionally used to perform this task.
A conventional multiple-beam antenna, known in the prior art, is illustrated in FIG. 1. Referring to FIG. 1, a series of antenna feeds 2 each generate a transmission beam signal which illuminates a focusing device 4, such as a lens or reflector. The focusing device 4 illustrated in FIG. 1 is a lens. The antenna feeds 2 are physically arranged along a focal arc and are positioned at various angles with respect to the normal to the focusing device to provide multiple contiguous beams in directions aligned with the vectors from feed centers to lens center. Typically, the antenna feed spacing along the focal arc is configured to establish beam crossover (overlap) at the half power (3 dB) point 6 of the beams. Also, typically, antenna feed width is chosen equal to the spacing between antenna feeds. As a result, the half-power (3 dB) beamwidth of each feed antenna is approximately equal to the included angle of the lens 4. Thus the lens illumination taper is only 3 dB.
The transmission beams typically contain a main signal lobe and one or more side lobes. To achieve suppression of side lobes from each transmission beam, a taper greater than 3 dB is required. In practice, for reasonable side lobe suppression, a taper of 12 dB or more is required. Referring to FIG. 1, each antenna feed element 2 is at least partially defined by a feed diameter, d. To achieve the desired illumination taper, the feed diameter of each element must be equal to twice the arc distance, d.sub.a, separating adjacent antenna feeds 2. This requirement dictates that adjacent antenna feeds either physically overlap or be spread apart to twice the angular separation, thus illuminating every other beam. However, configuring the array of FIG. 1 with twice the angular separation and half the beams would result in severe beam crossover losses.
The above-mentioned limitations were identified and explored in the article "Pattern Limitations in Multiple-Beam Antennas" by W. D. White, IRE Transactions on Antennas and Propagation, 430-436 (1962), which is hereby incorporated by reference. To overcome these problems, the White article discloses a beam combining network which provides for beam overlap and thus suppressed side lobes and low crossover losses. However, the beam combining network approach accomplishes this by introducing significant signal loss (terminations on combiners). White suggests that this loss can be masked by insertion of an amplifier between each feed element and the network. However, in such an approach, the amplifier must process multiple signals simultaneously (the signals from the adjacent overlapped beams). In signal transmission applications, this is a disadvantage because of the possibility of intermodulation distortion occurring between the multiple, high-level signals.
The White article also discusses an alternative arrangement which achieves beam overlap by supplying alternate beams from two separate antennas. This alternative arrangement uses an odd-beam antenna and an even beam antenna. This configuration has the obvious disadvantage of requiring twice as much apparatus and twice as much volume as compared to a single antenna array.